Various such belt type continuously variable transmissions and their control systems have been proposed and put into practical use. A typical belt type continuously variable transmission comprises a drive-pulley actuator, which is used to adjust the width of the drive pulley, and a driven-pulley actuator, which is used to adjust the width of the driven pulley, in axial thrust control. With these actuators, the axial thrusts applied on the drive pulley and the driven pulley are controlled to adjust the widths of the pulleys for controlling the speed change ratio of the transmission, which transmits the power of the engine to the drive wheels of a vehicle by the V belt disposed around the drive and driven pulleys.
FIG. 9 describes deviations Δ e of the center of the V belt 97 in its longitudinal direction in sectional views that pass through the axes of the drive pulley 98 and the driven pulley 95. Here, the surfaces of the V belt 97 in contact with the drive and driven pulleys 98 and 95 (hereinafter referred to as “V faces”) are formed linear in cross-sectional view. As shown in FIG. 9, while the speed change ratio of the transmission is varied from the minimum ratio (Low ratio) through the middle ratio (MID ratio) to the maximum ratio (OD ratio), the V belt 97 is being shifted in the direction of the axes of the drive and driven pulleys 98 and 95, which change their widths in relation to the speed change ratio. It should be noted that the amount or distance of the V belt 97 shifted on the drive pulley 98 and that on the driven pulley 95 are different such that the longitudinal center of the V belt 97 deviates laterally by Δe, resulting in the direction of the V belt 97 swinging laterally (this deviation of the longitudinal center of the V belt 97 is hereinafter referred to as “misalignment”).
While the V belt is misaligned, uneven abrasion occurs in the surfaces (V faces) of the V belt in contact with the drive and driven pulleys. Moreover, the above mentioned twisting of the V belt impairs the durability of the V belt. To alleviate or eliminate such misalignment of the V belt, drive and driven pulleys whose V faces are formed with a curve have been known (refer to, for example, Japanese Laid-Open Patent Publication No. 2002-31215).
However, if the curvature of the V faces of the drive and driven pulleys is excessive, the Hertz stress (compressive stress) at the part in contact with the V belt becomes large enough to cause a chipping or an abnormal abrasive wear in the V faces, resulting in a remarkable decrease in durability. In addition, if the V faces are defined by a complicated mathematical function, then the production of the drive and driven pulleys and the V belt becomes difficult.
Furthermore, if the distance that the points in contact between the V belt and the drive and driven pulleys travel while the speed change ratio is varied is not defined (this distance is hereinafter referred to as “contact lengths”), then optimal contact condition is not achieved. In this case, a contact can occur at an edge of the V faces of the V belt, and this condition leads to an abnormal abrasion and results in the transmission not achieving a sufficient range of speed change ratio.